U.S. Pat. Nos. 4,264,705; 4,427,759; 4,431,723; 4,442,302; 4,517,279; 4,540,649 and 4,716,094; Canadian Patent No. 1,267,475 and EP 335,247 describe multilayered elastomeric printing plates, flexographic printing plates and related photopolymerizable compositions used to prepare such plates. It is clear from these representative patents that there are many requirements for the preparation of acceptable printing plates, but chief among such requirements is that the quality of the photoimage be of the highest quality. That is, good resolution or a sharply defined image is essential along with durability and resistance to degradation or erosion by water or solvent based inks. EP 335m247 discloses that a hindered amine should also be present to provide stability to the cured product.
U.S. Pat. No. 4,216,019; EP 252,150 and WO 95/12148 describe the preparation of screen printing stencil compositions giving stencils with solvent/water resistance. These stencils are intended for use to give screen printing images of good resolution and quality.
The general method of using a stencil composition includes applying or coating a photosensitive stencil composition on a screen, drying the composition, exposing the stencil appropriately through a photomask to actinic radiation to form an image, optionally drying the image, treating the stencil with an alkaline developing solution or treating the dried stencil with an aqueous, alkaline ink and printing a substrate.
U.S. Pat. No. 4,517,279 teaches the use of high molecular weight butadiene/acrylonitrile copolymers with a selected carboxyl and acrylonitile content, an photopolymerizable ethylenically unsaturated monomer and photoinitiator as the photopolymerizable resin composition. U.S. Pat. No. 4,716,094 teaches the use of ethylenically unsaturated prepolymers, ethylenically unsaturated monomers, a photoinitiator and a surface tack modifier (to give a tack-free surface) as the photopolymerizable composition.
U.S. Pat. No. 4,666,821 discloses the preparation of hot melt solder masks using photopolymerizable compositions.
U.S. Pat. No. 4,824,765 describes the use of water-soluble photoinitiators in general photoimaging processes useful to prepare printing plates, screen printing stencils, solder masks, litho and letterpress printing plates, etch resists, UV cured inks, overcaoting lacquers and abrasion or mar resistant coatings.
U.S. Pat. No. 5,501,942 give a typical procedure for the preparation of solder masks or resists which include applying a photosensitive composition to a substrate, removing water from the composition to form a photosensitive film on the substrate, exposing the coated substrate with actinic light in a desired pattern, removing the unexposed areas of said coating with an aqueous or aqueous-alkaline solution to uncover the substrate in the non-exposed areas, and subjecting the coating, on the substrate to a thermal or option UV cure.
EP 261,910 and EP 295,944 both describe the water developable photosensitive resin plate suitable for making relief printing plates with high resistance to water-based inks.
From each of these references and the art generally, it is clear that steps for producing printing plates, solder masks or any other of the end-use application mentioned above involve a photopolymerizable composition having the following components:
(a) a polymer or prepolymer containing, some polymerizable or crosslinkable groups; PA1 (b) an ethylenically unsaturated monomer; PA1 (c) a photoinitiator or light activated cationic catalyst; and optionally PA1 (d) other compounds aimed at modifying some aspect of the ultimate polymerized or cured final product. PA1 (A) from 5 to 98% by weight, based on the total weight of the components (A), (B), (C) and (D), of a polymer selected from the group consisting of a prepolymer, a binder polymer and a mixture thereof, PA1 (B) from 1.0 to 94% by weight, based on the total weight of components (A), (B), (C) and (D), of an ethylenically unsaturated monomer or mixture thereof, PA1 (C) from 0.001 to 10% by weight, based on the total weight of components (A), (B), (C) and (D), of a photopolymerization initiator selected from the group consisting of acetophenone and a derivative thereof, benzoin and a derivative thereof, benzophenone and a derivative thereof, anthraquinone and a derivative thereof, xanthone and a derivative thereof, thioxanthrone and a mixture thereof, and a mixture of one or more of these initiators, and PA1 (D) from 0.0001 to 0.2% by weight, based on the total weight of components (A), (B), (C) and (D), of a polymerization inhibitor selected from the group consisting of nitroxyl radicals, the quinone methides, phenothiazine, hydroquilnones, selected phenols, galvinoxyl and nitroso compounds. PA1 4-benzylidene-2,6-di-tert-butyl-cyclohexa-2,5-dienone; or PA1 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine.
These references note that actinic radiation is used to polymerize the liquid photopolymerizable resin composition for making the stencil, printing plate, etch resist or solder mask, but none of the references teaches or suggests that the quality or the stability of the photoimage can be greatly enhanced by inclusion ol a polymerization inhibitor in the photopolymerizable resin composition
The photoimaging technique is used in many fields such as printing plates, etch resists and solder mask applications. Photomasks have always been used in such applications to transfer an image onto a light sensitive or photopolymerizable material. The process begins with the irradiation of actinic light onto the light sensitive material placed under a photomask. Where the incident light passes through the transparent or open part of the photomask onto the light sensitive material, said material hardens via polymerization or crosslinking steps. Where the irradiated light is blocked by the photomask, the light sensitive material beneath the photomask should receive no incident actinic light and should remain in an unpolymerized state. During development of said photoimage, the unpolymerized material is then removed from the polymerized material which latter material becomes the photoimage.
Theoretically, such an image should be clear, sharp and of the highest quality. However, in practice there is a problem caused by scattering of the light while it passes through the photomask or where the laser beam impinges causing said light to diffuse out when it exits the photomask. This result is caused by the fact that any photomask has a finite thickness leading to some diffusion of incident light. Also, impurities or fillers in the formulation itself may cause some scattering of the incident light. The diffused incident light causes some polymerization to occur outside of the discrete dimensions of the open portions of the photomask where no polymerization is desired. In the case of printing plates, the unwanted polymerization will cause the reverse image to fill in with excessive polymer so that the printed image will lose its definition and clarity. In the case of electron applications, the unwanted polymerization will fill the space between two lines also reducing the resolution of the desired image.
It is noted that the instant process pertains to both positive and negative resists since each type of resists requires high resolution images.